1. Field of the Invention
The present invention relates to a method of producing milk-fermented food, and more particularly, to a method of producing milk-fermented food by fermenting milk with bifidobacterium and/or lactic acid bacteria, and also to milk-fermented food containing bifidobacteria and/or lactic acid bacteria.
"Milk" herein means whole milk or skimmed milk obtained from cows, goats or like animals, or reconstituted milk prepared from powdered milk made from these two kinds of milk, or a mixture of all the kinds of milks mentioned above. "Fermented food" includes beverages such as lactic acid bacteria beverages and those beverages processed by heating and sterilization after fermentation. "Acidity" means volume(ml) of 0.1 N-NaOH per ml to neutralize 10 ml of sample.
2. Description of the Prior Art
Bifidobacterium is one of the most dominant bacterial flora in the intestine of infants and most healthy adults and seems to contribute to the maintenance of a normal flora of intestine. It has been reported that the administration of these live bacteria to human or domestic animals suffering from diarrhea or patients infected with superinfection remarkably improves their symptom. Accordingly, yogurt, milk beverages and sweets which contain live Bifidobacteria have been developed and also have been commercially available in order to improve people in their own health by allowing regular ingestion of such bifidobacteria.
Bifidobacterium is, however, originally anaerobic and thus readily affected by oxygen and easily destroyed in a milk-fermented product having high acidity. Lactic acid bacteria has a tendency to die out, but not so readily as bifidobacterium when pH falls during fermentation.
In milk-fermented food products, It is necessary to maintain the numbers of viable cell at a high level at least for two or more weeks, however, bifidobacteria and lactic acid bacteria will die out because of the decrease in pH caused by their own generation of organic acids. Such qualities limit their acidity to a low level in the milk-fermented food obtained by use of the bacteria. Also, it is difficult to maintain high levels of viable cell count of the product for long period even at moderately suppressed acidic condition. Therefore, a more improved taste cannot be pursued by decreasing the pH of the food. High numbers of viable cells are also difficult to maintain for long periods when the acidity is suppressed in milk-fermented food.
In addition, bifidobacteria requires some growth promoters to obtain sufficient growth in pure milk-medium. Conventionally, there have been growth promoters such as yeast extract, sulphur content amino acids (for example, cystein, methionine and the like ), soybean peptide (refer to Monthly Food Chemical, 8, 64, 1988) and so on. In many cases these substances cannot be used in sufficient amounts because of their own characteristic taste.
The growth accelerating effect of soybean milk is disclosed in Japanese Patent Publication Nos. 45-9822, 51-14256 and 55-89390, however, soybean milk has its own peculiar smell and thus changes for the worse in taste are unavoidable when sufficient amounts of soybean milk are used.
A method is disclosed in Japanese Patent Publication No. 59-179064, wherein soybean milk was neutralized by calcium hydroxide in the presence of calcium dichloride after the removal of its protein by the addition of phosphoric acid or phosphoric acid salts, and then it was precipitated by heating and desalting, thereby condensing the fraction of growth promoting substance for bifidobacteria contained in the soybean milk. The fraction of the growth promoting substance for bifidobacterium thus obtained was composed of 7.2% of crude protein, 77.3% of carbohydrate and 15.5% of crude ash, the carbohydrate was composed of 51.9% of sucrose, 27.0% of stachyose and 11.2% of raffinose. It has been also reported that a highly activated growth promoting substance for bifidobacterium was obtained by purifying this fraction with the use of a membrane separation method and, which was made of 12.3% of raffinose, 84.5% of stachyose and 3.2% of verbacose (Japanese Patent Publication No. 60-66978). Japanese Patent Publication No. 62-155082 disclosed a growth promoting substance for bifidobacterium which was directly extracted from defatted soybean meal by using a 20 to 60 w/v % of alcoholic solution. The main component of the accelerator was an oligosaccharide which was composed of 82.5% to 85.1% of carbohydrates, 6.9% to 7.5% of protein, 1.0% to 2.8% of lipid and 6.3 % to 6.4% of ash. These bifidobacterium accelerators mainly made up of oligosaccharides are so difficult to digest that they go down to a lower area of the intestines, thereby effectively stimulating the growth of bifidobacterium in an entero carbon source-deficient environment. These oligosaccharides are, however, not so effective for stimulating the growth of bifidobacterium in a culturing process of producing milk-fermented food because of the simultaneous presence of other readily available saccharides, lactose or glucose or so on.
Various efforts have been exerted to shorten the fermenting period when employing a lactic acid bacterium, accordingly, more effective means have been expected to emerge.